Mobile communication is increasingly advancing in popularity while digital processing has enabled a rapid migration of mobile wireless services from analog to digital communications. More and more, the cellular service providers are focusing on techniques for high-capacity communication of digital information over wireless links.
In 1998 the Chinese Wireless Telecommunications Standards proposed to the International Communications Union a new standard that is based on Time Division Duplexing (TDD) and Synchronous Code Division Multiple Access (CDMA) technology (TD-SCDMA) for TDD. The International Communications Union has approved and adopted this proposal. The adopted system has several advantages over 2nd generation and other 3rd communication systems.
The TDD uses a single frequency band for uplink as well as downlink, however, uplink and downlink occur at different predetermined time slots. The CDMA is based on Direct-Sequence Spread-Spectrum (DS-SS) principles, where multiple users simultaneously occupy the same radio frequency channel, separated only by user-specific spreading or signature sequences. As a result, in TD-SCDMA system, time slots and spreading codes separate the users in a cell. TD-SCDMA also includes support for innovative use of key technologies such as smart antennas, joint detection, and dynamic channel allocation to achieve near optimal performance.
DS-SS communication requires detection of one or more spreading chip-code sequences embedded in an incoming spread-spectrum signal as well as subsequent synchronization of the receiver with the detected chip-code sequence. Also, prior to transmission, predetermined symbols (e.g., midambles) are inserted in each frame to detect and compensate for the distortion of the information symbols by comparing the received symbols to the predetermined symbols. In other words a transmitter inserts, what is called, training symbols in each frame, and a receiver, which already expects these training symbols, extracts the distorted symbols from the received frames and uses their distortion information for channel estimation.
In a CDMA environment, as well as other types of communication employing DS-SS, two or more transmitters may transmit at the same time using different spreading codes. In such a situation, particularly if the receiver must receive the transmissions simultaneously, the receiver must search for and acquire multiple codes at the same time from within a broad-spectrum wireless signal. In a CDMA system, the multiple access interference (MAI) affects all users equally. While detection schemes such as the Rake receiver are sub-optimal because they only consider the user's signal information without any attempt to characterize the interference from other users, the joint detection algorithms process all users of the cell in parallel which includes the interference information from all.
Joint detection schemes are complex and computationally intensive.
Complexity grows exponentially as the number of codes increases. Therefore, joint detectors are not suitable for use in other CDMA systems because of the high number of codes used in those systems. However, joint detection and its associated parallel processing are well suited for TD-SCDMA systems because in every time slot the users are synchronized and are limited to a very manageable number. The result is a joint detector of reasonable complexity that can easily be implemented in today's parallel computational architectures. However, the accuracy of the detected signals in a joint detector is directly related to the quality of its channel estimation, which is an essential part of a joint detector. In general, the more accurate the channel estimation, the better the system performance.